Gas turbine engines include a compressor for compressing air that is mixed with fuel and ignited in a combustor for generating combustion gases. The combustion gases from the combustor flow to a turbine that extracts energy for driving a shaft to power the compressor and produces output power, often for powering an electrical generator.
Increased requirements for low emissions from turbine power plants now require low rates of emissions of NOx (mono-nitrogen oxides NO (nitric oxide) and NO2 (nitrogen dioxide)), CO (carbon monoxide) and other pollutants from turbine combustors.
Conventional turbine combustors use non-premixed diffusion flames, where fuel and air freely enter the combustion chamber separately and mixing of the fuel and air occurs simultaneously with combustion, and where resulting flame temperatures typically exceed 4000° F. with NG, LF or syngas fuels, so as to produce relatively high levels of NOx emissions. Thus, temperatures in combustion chamber primary zones can get very high if water is not injected, although temperatures do drop along the length of the combustion chamber. Water is generally used because a diffusion flame is used in these combustors and primary zone temperatures are very high and produce NOx as much as approximately 250 ppm with syngas/LF fuels and approximately 120 ppm with NG fuel if water is not used.
Approximately 95% of the combustor exiting NOx, which is measured in ppmvd (parts per million, volumetric dry) @15% O2, has already been formed before the combustion gases reach the dilution holes in a conventional combustor liner. NOx formation rates are highest in a narrow zone of the combustion chamber, and become very much less so after the combustion gases reach the dilution holes in the conventional combustor liner. Thus, air introduced by dilution holes in a conventional combustor liner does not participate in a reduction of combustion gases' temperatures and NOx production.
As is explained in the background section of U.S. Pat. No. 6,192,689, one method commonly used to reduce peak temperatures in conventional diffusion flame combustors, and thereby reduce NOx emissions, is to inject water or steam into the combustor. However, water or steam injection is a relatively expensive technique and can cause the undesirable side effect of quenching (i.e., rapid cooling) carbon monoxide (CO) burnout reactions, and which is limited in its ability to achieve low levels of pollutants.
Conventional diffusion flame combustors are effective for burning natural gas (NG), synthesis gas (syngas) and liquid fuels (LF) in low megawatt (MW) turbine machines. But conventional combustors use a very old liner cooling design that involves the use of water or steam injection, which is not desirable in gas turbine power plants from life of components, operability and cost of electricity perspectives. Sufficient efforts have not been made to reduce water consumption in these machines.